Optical sensing technology has been used to locate and track movement of objects in two and three dimensions. U.S. patent application Ser. No. 12/327,511, filed Dec. 3, 2008, and entitled “Method of Location an Object in 3D” and U.S. patent application Ser. No. 12/435,499, filed May 5, 2009, and entitled “Optical Distance Measurement by Triangulation of an Active Transponder” provide examples and details regarding how optical sensing technology may be used to locate and track objects. The contents of both these patent applications are incorporated by reference herein.
Some optical systems locate and track objects by placing one or more light sources in a first object and one or more position sensitive light detectors in a second object. The location of the first object relative to the second object may then be calculated using triangulation or other mathematical calculations based on the detected position of light from the light source(s) directly striking the detector(s). These optical systems may be limited to tracking objects equipped with either a complementary light source or detector. Existing medical devices such as optical heart rate monitors and blood oxygen level measurement devices use a light source, light detector, and simple photodetector geometry to calculate heart rates or measure blood oxygen levels.
These existing optical pulse oximeters and heart rate monitors work by having a user place a transparent body part, such as a fingertip or earlobe between the light sources and detector(s). As the arterial blood vessels expand and contract with each heartbeat, the amount of light flowing through the body part changes. A user's heartbeat can be measured based on the change in light detected at the detector. Different colors of light are used to measure blood oxygen level since absorbance of oxygenated and deoxygenated blood varies at different colors. In blood oxygen monitors, “locking” measurements to the heartbeat signal may allow some rejection of interference signals from stagnant blood outside the arteries.
In order for these existing pulse oximeters and heart rate monitors to provide reliable results, manufacturers have placed the light sources and detectors flush or close to the transparent body part. This was done to prevent ambient light from reaching the detector, which caused signal interference and inaccurate results. Light sources and detectors were often placed close the body part by a mechanical device, such as a clip or spring, which also requires additional maintenance. Manufacturers have also tried to reducing the effects of other sources of error leading to inaccurate results, such as movements of the body part during heart rate/photoplethysmograph (PPG) and oximetry measurements, by implementing various algorithms to “guess” and reduce errors caused by body part movements.
Another optical object location and tracking technique is used in some optical mice. In this traditional technique, light is emitted from a light source in the bottom of the object, in this case a computer mouse; reflected off the surface of another object, such as desktop or mouse pad; and detected by a relatively small pixel count CMOS camera whose output when coupled with optical flow algorithm produces accurate velocity measurement. This existing technique, however, is sensitive to interference from ambient light and cannot be used in environments where interfering light from outside sources can reach the detector.
There is a need for an optical position and movement tracking device that can track objects unequipped with a complementary light source or detector without being affected by interference from ambient light. This need applies to both optical mouse applications as well as to measurements of medical information, such as PPG. There is also a need to integrate position tracking information with the medical measurements so that movement errors from body part movements can be directly removed from PPG data instead of through “guessing” algorithms. There is also a need to use position measurement information to guide a user in repositioning their body part to an optimal location for measurement. There is also need for performing each of these functions in a “reflection mode” where light emitted from a source is reflected off the object or body part and detected at a detector in order to avoid mechanical design and maintenance issues associated with placing an object between a light source and detector or affixing a light source or detector to the object.